User is another word for a person. But when we use that word to describe someone else, we're likely implying that they're using the thing we're making. It could be a website, a product or service, a grocery store, a museum exhibit, or anything else people interact with.

We're full of contradictions. We're known to exhibit strange behaviors. From how we use mobile phones to how we traverse grocery stores, none of us are exactly the same. We don't knowwhy we do what we do. We don't really knowwhy we like what we like, but we do know it when we see it. We're fickle.

We expect things to be digital, but also, in many cases, physical. We want things to feel auto-magic while retaining a human touch. We want to be safe, but not spied on. We use words at our whim.

Postal codes are what we call an exactclassification. We can generally rely on the codes to hold steady. If the postal code is 10012, the building is in Manhattan. There's nothing to argue about. It just is.

For example, in editing this book, Nicole suggested I use the term "Postal code" instead of "Zip code" in the example above. Both would have expressed the point, but one is more exact for our context, which includes readers outside of the United States.

For example, what if I had organized the lexicon in the back of this book by chapter, instead of alphabetically? This might be an interesting way of arrangingthings, but it would need to be explained, so you could find a term.

Because there are many words for the same thing, exactclassifications can slow us down. For example, I recently tried to buy some zucchini at a grocery store. But it wasn't until the clerk in training found the code for "Squash, Green" that she could ring me up.

Because your structure may change a hundred times before you finish making it, you can save time and frustration by thinking with boxes and arrows before making real changes. Boxes and arrows are easier to move around than the other materials we work with, so start there.